Electroluminescent devices are a promising technology for flat-panel displays and area illumination lamps. These devices rely upon thin-film layers of materials coated upon a substrate, and include organic, inorganic and hybrid inorganic-organic light-emitting diodes. The thin-film layers of materials can include, for example, organic materials, quantum dots, fused inorganic nano-particles, electrodes, conductors, and silicon electronic components as are known and taught in the LED art.
Irrespective of the particular electroluminescent device configuration tailored to these broad fields of applications, all electroluminescent devices function on the same general principles. An electroluminescent (EL) unit is sandwiched between two electrodes. At least one of the electrodes is at least partially light transmissive. These electrodes are commonly referred to as an anode and a cathode in analogy to the terminals of a conventional diode. When an electrical potential is applied between the electrodes so that the anode is connected to the positive terminal of a voltage source and the cathode is connected to the negative terminal, the LED is said to be forward-biased. Positive charge carriers (holes) are injected from the anode into the EL unit, and negative charge carriers (electrons) are injected from the cathode. Such charge carrier injection causes current flow from the electrodes through the EL unit. Recombination of holes and electrons occurs within the light-emitting layer and results in emission of light. For example, electroluminescent devices containing quantum dot light-emitting diode (LED) structures can be either inorganic or hybrid inorganic-organic, and the recombination of the holes and electrons occurs within the core of a quantum dot in the light-emitting layer. A hybrid inorganic-organic EL unit can be formed of a stack of sublayers that can include small-molecule layers or polymer layers.
In the late 1990's LED devices containing mixed emitters of organics and quantum dots were introduced (Mattoussi et al., Journal of Applied Physics 83, 7965 (1998)). Quantum dots are light-emitting, nano-sized, semiconductor crystals. Adding quantum dots to the emitter layers could enhance the color gamut of the device; red, green, and blue emission could be obtained by simply varying the quantum-dot particle size; and the manufacturing cost could be reduced. Because of problems such as aggregation of the quantum dots in the emitter layer, the efficiency of these devices was rather low in comparison with typical OLED devices. The efficiency was even poorer when a neat film of quantum dots was used as the emitter layer (Hikmet et al., Journal of Applied Physics 93, 3509 (2003)). The poor efficiency was attributed to the insulating nature of the quantum-dot layer. Later the efficiency was boosted (to ˜1.5 cd/A) upon depositing a mono-layer film of quantum dots between organic hole and electron transport layers (Coe et al., Nature 420, 800 (2002)). It was stated that luminescence from the quantum dots occurred mainly as a result of Forster energy transfer from excitons on the organic molecules (electron-hole recombination occurs on the organic molecules). Regardless of improvements in efficiency, these hybrid devices still suffer from all of the drawbacks associated with pure OLED devices.
Recently, a mainly all-inorganic LED was constructed (Mueller et al., Nano Letters 5, 1039 (2005)) by sandwiching a monolayer thick core/shell CdSe/ZnS quantum-dot layer between vacuum deposited inorganic n- and p-GaN layers. The resulting device had a poor external quantum efficiency of 0.001 to 0.01%. Part of that problem could be associated with the organic ligands of trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP) that were reported to be present post growth. These organic ligands are insulators and would result in poor electron and hole injection into the quantum dots.
As described in co-pending, commonly assigned US Publication 2007/0057263 by Kahen, which is hereby incorporated by reference in its entirety, additional semiconductor nanoparticles may be provided with the quantum dots in a layer to enhance the conductivity of the light-emitting layer.
OLED devices generally can have two formats known as small molecule devices, such as disclosed in U.S. Pat. No. 4,476,292, and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device may include, in sequence, an anode, an organic EL element, and a cathode. In most designs, one of the electrodes is reflective and the other transparent. The organic EL element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), a light-emissive layer (LEL), and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the LEL layer. Tang et al. (Applied Physics Letters, 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved.
Light is generated in an LED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the respective charge-transport layers and recombine in the emissive layer. Many factors determine the efficiency of this light-generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of LEL can determine how efficiently the electrons and holes are recombined and result in the emission of light, etc.
It has also been found, that one of the key factors that limits the efficiency of LED devices is the inefficiency in extracting the photons, generated by the electron-hole recombination, out of the LED devices. Due to the high optical indices of the materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the LED devices and make no contribution to the light output from these devices. Because light is emitted in all directions from the internal layers of the LED, some of the light is emitted directly from the device, and some is emitted into the device and is either reflected back out or is absorbed, and some of the light is emitted laterally and trapped and absorbed by the various layers comprising the device. In general, up to 80% of the light may be lost in this manner.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer and transparent cover. Light generated from the device is emitted through the top transparent electrode and transparent cover. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
In any of these LED structures, the problem of trapped emitted light and reflected ambient light remains. Referring to FIG. 8, a bottom-emitting LED device as known in the prior art is illustrated having a transparent substrate 10, a transparent first electrode 12, an EL unit 14 which contains a light-emitting layer, a reflective second electrode 16, a gap 19 and a cover 20. The gap 19 is typically filled with desiccating material. Light emitted from the EL units 14 can be emitted directly out of the device, through the transparent substrate 10, as illustrated with light ray 1. Light may also be emitted and internally guided in the transparent substrate 10 and EL unit 14, as illustrated with light ray 2. Additionally, light may be emitted and internally guided in the EL unit 14, as illustrated with light ray 3. Light rays 4 emitted toward the reflective electrode 16 are reflected back toward the substrate 10 and follow one of the light ray paths 1, 2, or 3. Ambient light 6 incident on the LED may be reflected from the reflective electrode 16, thereby reducing the ambient contrast of the LED device. In some prior-art embodiments, the electrode 16 may be opaque and/or light absorbing. Such an arrangement will increase the contrast by absorbing ambient light, but also absorbs the light 4 emitted toward the electrode 16. The bottom-emitter embodiment shown may also be implemented in a top-emitter configuration with a transparent cover and top electrode 16.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. Such techniques include the use of diffraction gratings, brightness enhancement films having diffractive properties, reflective structures, and surface and volume diffusers. The use of micro-cavity techniques is also known. However, none of these approaches cause all, or nearly all, of the light produced to be emitted from the device. Moreover, diffractive techniques cause a significant frequency dependence on the angle of emission so that the color of the light emitted from the device changes with the viewer's perspective. Scattering techniques are also known and described in, for example, US 2006/0186802 entitled “OLED device having improved light output” by Cok, which is hereby incorporated by reference in its entirety.
US 2005/0007000 entitled, “Brightness and contrast enhancement of direct view emissive displays” describes emissive displays including a plurality of independently operable light emitters that emit light through one or more transmissive layers. The emissive displays further include elements disposed between the light emitters and the transmissive layers to frustrate total internal reflections that can occur at one or more of the interfaces created by the transmissive layers, such as at an interface between the light emitter and a transmissive layer or at an interface between a transmissive layer and air. By frustrating total internal reflections, the brightness of the emissive display can be enhanced. Elements for frustrating total internal reflections include volume diffusers, surface diffusers, microstructures, and combinations of these or other suitable elements.
Scattering techniques are known to assist in extracting light from LED devices. Chou (WO 02/37580) and Liu et al. (U.S. Publication No. 2001/0026124) taught the use of a volume or surface scattering layer to improve light extraction. The scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has an optical index that matches these layers. Light emitted from the OLED device at an angle higher than a critical angle, and would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device. The efficiency of the OLED device is thereby improved but still has deficiencies as explained below. Moreover, the contrast of the device is not improved under diffuse illumination.
U.S. Pat. No. 6,787,796 entitled “Organic electroluminescent display device and method of manufacturing the same” by Do et al issued Sep. 7, 2004, describes an organic electroluminescent (EL) display device and a method of manufacturing the same. The organic EL device includes a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, wherein a light-loss preventing layer having different refractive index areas is formed between layers of the organic EL device having a large difference in refractive index among the respective layers. U.S. Publication No. 2004/0217702 entitled “Light extracting designs for organic light emitting diodes” by Garner et al., similarly discloses use of microstructures to provide internal refractive index variations or internal or surface physical variations that function to perturb the propagation of internal waveguide modes within an OLED. When employed in a top-emitter embodiment, the use of an index-matched polymer adjacent the cover is disclosed.
Light-scattering layers used externally to an OLED device are described in U.S. Publication No. 2005/0018431 entitled “Organic electroluminescent devices having improved light extraction” by Shiang and U.S. Pat. No. 5,955,837 entitled “System with an active layer of a medium having light-scattering properties for flat-panel display devices” by Horikx, et al. These disclosures describe and define properties of scattering layers located on a substrate in detail. Likewise, U.S. Pat. No. 6,777,871 entitled “Organic ElectroLuminescent Devices with Enhanced Light Extraction” by Duggal et al., describes the use of an output coupler comprising a composite layer having specific refractive indices and scattering properties. While useful for extracting light, this approach will only extract light that propagates in the substrate (illustrated with light ray 2) and will not extract light that propagates through the organic layers and electrodes (illustrated with light ray 3).
In any case, scattering techniques, by themselves, cause light to pass through the light-absorbing material layers multiple times where they are absorbed and converted to heat. Moreover, trapped light may propagate a considerable distance horizontally through the cover, substrate, or organic layers before being scattered out of the device, thereby reducing the sharpness of the device in pixellated applications such as displays. For example, as illustrated in FIG. 9, a prior-art pixellated bottom-emitting LED device may include a plurality of independently controlled sub-pixels 50, 52, 54, 56, and 58 and a scattering layer 22 located between the transparent first electrode 12 and the substrate 10. A light ray 5 emitted from the light-emitting layer may be scattered multiple times by scattering layer 22, while traveling through the substrate 10, EL unit(s) 14, and transparent first electrode 12 before it is emitted from the device. When the light ray 5 is finally emitted from the device, the light ray 5 has traveled a considerable distance through the various device layers from the original sub-pixel 50 location where it originated to a remote sub-pixel 58 where it is emitted, thus reducing sharpness. Most of the lateral travel occurs in the substrate 10, because that is by far the thickest layer in the package. Also, the amount of light emitted is reduced due to absorption of light in the various layers.
U.S. Publication No. 2004/0061136 entitled “Organic light emitting device having enhanced light extraction efficiency” by Tyan et al., describes an enhanced light-extraction OLED device that includes a light-scattering layer. In certain embodiments, a low-index isolation layer (having an optical index substantially lower than that of the organic electroluminescent element) is employed adjacent to a reflective layer in combination with the light-scattering layer to prevent low-angle light from striking the reflective layer, and thereby minimize absorption losses due to multiple reflections from the reflective layer. The particular arrangements, however, may still result in reduced sharpness of the device and does not improve contrast.
It is known to improve the contrast of a LED device by employing, for example, black-matrix materials between the light-emitting areas or by using color filters. While such methods are useful, the presence of a reflective electrode still decreases the ambient contrast significantly. As noted above, circular polarizers may be employed, but Applicants have determined that light-extraction techniques such as scattering layers tend to be incompatible with such polarizers.
As taught in the prior art, classic scattering theory employs arrays of spheres. If such spheres are arranged adjacent to each other to form a layer, the ratio of the volume of the spheres (Vp=(4*π*r3)/3) divided by the volume of the layer (VL=(2*r)3) is equal to π/6 or 0.5236. U.S. Pat. No. 5,955,837, entitled, “Electroluminescent illumination system with an active layer of a medium having light-scattering properties for flat-panel display devices” describes the use of a half-monolayer of scattering particles on the substrate of a bottom-emitting electro-luminescent device. The volume ratio of such a layer is at most one half of a classical mono-layer array of contacting spheres, or approximately 0.26. While optimizing the combination of extraction of light trapped in the substrate and non-scattering of non-trapped light for the disclosed devices, such a material layer may not scatter light trapped in the organic and electrode layers, and is difficult to form. Moreover, as the amount of trapped light relative to the amount of emitted light increases, the relative amount of desirable scattering increases so that the described half-monolayer of scattering particles may not optimally extract all of the emitted light from LED devices of interest. It is also true that the organic electro-luminescent materials, reflective electrodes, and transparent electrodes all absorb some light. Hence, it can be desirable to scatter trapped high-angle light as soon as possible to minimize absorption. Additionally, experiments performed by applicant have shown that such volume ratios of a practical scattering layer does not extract light optimally for some LED device structures.